This invention relates to the processing of soy flour. Specifically, the invention is a process for the treatment of soy flour to obtain a soy flour protein concentrate.
Soybeans are widely cultivated and are known to be an excellent source of relatively inexpensive high-quality proteins. Soybean protein is often concentrated or extracted from soybeans and used in a variety of food products. This is generally done by milling the soybeans and removing the naturally occurring soybean oils to give a soy flour. This flour is then subjected to a treatment process to produce a soy protein concentrate.
Soy flour treatment processes fall into two general categories: The aqueous extraction methods and the nonaqueous extraction methods.
The non-aqueous methods rely upon organic solvents in order to separate the protein component from the nonprotein component. Organic solvents have an undesirable effect upon the protein. Most notably, they cause serious denaturation of the proteln as measured by the Nitrogen Solubility Index (NSI) (American Oil Chemist's Method Ba 11-65).
Highly denatured proteins are characterized by very low NSI's, often as low as 5. Denatured proteins have many undesirable properties including poor heat gelability, water binding and heat coagulation. These proteins also have diminished emulsifying capabilities and are less palatable than undenatured proteins.
Aqueous extraction techniques generally result in protein products which have a NSI of at least 65. Soy protein extracts having an NSI of 65 or higher are quite palatable and have good heat gelability, water binding and heat coagulation properties.
The aqueous extraction teohniques are generally based on the work of Sair as illustrated in his U.S. Pat. No. 2,881,076 and are also known as "acid-leach" methods.
The aqueous methods take advantage of the insolubility in water at their iso-electric point of the glycinin proteins which are found in soy flour. Typically, an aqueous suspension of soy flour is brought to a pH of about 4.0 to 4.8 (the iso-electric range of the glycinin soy protein) and the insoluble protein is precipitated while a large portion of the soy flour remains ln solution. The protein-rich precipitate can then be separated from the supernatant, yielding a high-quality protein concentrate.
The various acid-leach methods which have been known in the past suffer from a variety of process drawbacks. Most notably, they require large, specially constructed holding or mixing tanks. Usually, these extractions are conducted in a batch-type sequence. In these cases, the soy flour, the water and the acidifying agent are mixed in stainless steel or glass-lined tanks which are acid resistant and suitable for food handling.
The batch procedures have been used for a number of reasons. One of the most important of these reasons is to allow for thorough mixing of the soy flour and water before and during acidification. Without vigorous mixing, an unworkable soy flour paste will be formed when a large quantity of soy flour is added at once to water. Also, during acidification, the soy protein acts as a strong buffer and will result in significant local variations in pH unless strong agitation is provided. These local variations in pH can cause incomplete separation of the protein component from the non-protein component, as well as partial denaturation of the protein. When these processes are conducted on a commercial scale, these stainless steel or glass lined mixing tanks must be quite large. In fact, tanks capable of holding tens of thousands of gallons of solution are often needed.
The size of the stainless steel or glass lined tanks used in aqueous soy protein extraction systems poses serious problems. First, tanks of this type are expensive to construct and maintain. Thus, the large tanks used in aqueous extractions represent a formidable capital outlay. Second, tanks of this size are difficult to house and require the construction of large plant structures. These structures also represent large capital expenditures.
Another significant drawback of the previously known aqueous extraction technologies is the time required to complete the protein extraction process. The basic Sair process, as described in U.S. Pat. No. 2,881,076, requires approximately 27 hours to process a single batch of soy flour. Such delays are not only undesirable from an economic standpoint, but are also known to result in a diminution of protein quality.
One recent attempt to solve the problems associated with aqueous soy protein extractions is the work of Sailer as described in U.S. Pat. No. 4,410,554. Sailer shows that by the use of certain procedures, a semi-continuous extraction process which has a pass-through time of no more than one hour can be achieved. However, Sailer still requires several large holding tanks, preferably two 10,000 gallon tanks one 3,000 gallon tank and six 1,000 gallon tanks. Each tank must have a powerful agitator.